1. Field of the Invention
The present invention relates to a method of forming a buffer layer for a nitride compound semiconductor light emitting device and a nitride compound semiconductor light emitting device having the buffer layer, and more particularly, to a method of forming a buffer layer for a nitride compound semiconductor light emitting device and a nitride compound semiconductor light emitting device having the buffer layer, wherein a sapphire (Al2O3) substrate is annealed in a state where a nitrogen source gas is introduced so as to form an AlN compound layer on the sapphire substrate, and a semiconductor layer is then formed on the AlN compound layer, thereby reducing a crack or the like that may be generated due to differences in lattice constant and thermal expansion coefficient between the substrate and the semiconductor layer.
2. Discussion of the Background
Generally, since a nitride compound semiconductor containing an element of Group III, such as GaN and AlN, have excellent thermal stability and a direct transition type energy band structure, they have been recently spotlighted as materials of photoelectronic devices in a blue light region and an ultraviolet light region. Specifically, blue or green light emitting devices using GaN have been used in various applications such as large-sized natural color flat-panel displays, traffic signal lights, indoor illumination, high-density light sources, high-resolution output systems, and optical communication.
A nitride compound semiconductor containing an element of Group III is grown on a heterogeneous substrate, which is made of sapphire, silicone carbide or the like having the structure of a hexagonal system, by using a process such as metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). “Heterogeneous substrate” as used herein means a substrate made of a material different from upper semiconductor layers grown on the substrate. If a layer of a nitride compound semiconductor containing an element of Group III is formed on the heterogeneous substrate, a crack or warpage and thence a dislocation may be generated in the semiconductor layer due to differences in lattice constant and thermal expansion coefficient between the semiconductor layer and the substrate. The crack, warpage and dislocation in the semiconductor layer deteriorate properties of a light emitting device. Therefore, in order to reduce a stress that may be generated due to the differences in lattice constant and thermal expansion coefficient between the substrate and the semiconductor layer, a buffer layer has been typically used.
FIG. 1 is a sectional view illustrating a conventional method of forming a buffer layer for a nitride compound semiconductor light emitting device.
Referring to FIG. 1, a buffer layer 2 is formed on a substrate 1. The buffer layer 2 is formed of AlxGa1-xN (0≦x≦1) by using the MOCVD or MBE process.
Upon formation of the buffer layer 2, trimethyl aluminum (TMAl; Al(CH3)3) and trimethyl gallium (TMG; Ga(CH3)3) are used as source gases for Al and Ga, respectively, while NH3 is used as a reaction gas. After the source gases and the reaction gas are introduced into a reaction chamber, the buffer layer 2 is formed at a temperature of 400° C. to 800° C.
Then, the temperature of the reaction chamber is increased to 900° C. to 1200° C., and a GaN-based semiconductor layer 3 having P-N junction is formed on the buffer layer 2. Thereafter, electrodes are formed on the semiconductor layer 3 to fabricate a light emitting device.
According to the prior art, the formation of the buffer layer 2 between the semiconductor layer 3 and the substrate 1 can reduce a crack that may be generated due to the differences in lattice constant and thermal expansion coefficient difference between the substrate 1 and the semiconductor layer 3.
However, since the buffer layer 2 that has been grown at a lower temperature is generally grown in a direction perpendicular to the substrate, the buffer layer 2 has a columnar structure. Further, since an AlxGa1-xN (0≦x≦1) crystal has a relatively strong binding force between atoms within the crystal, a large interface space exists between columns within the columnar structure and a crystal stress is great. Meanwhile, the semiconductor layer 3 is influenced by the crystal structure, crystalloid and column size distribution of the buffer layer 2 that is positioned below the semiconductor layer 3. That is, crystal defects in the buffer layer 2 are transferred to the semiconductor layer 3 positioned above the buffer layer 2. Therefore, crystal defects depending on the columnar structure of the buffer layer 2 are exhibited as crystal defects in the semiconductor layer 3.